1. Technical field of the Invention
The present invention relates to a drive circuit for a switching element, and in particular, to a drive circuit for a switching element including a constant-current control unit that performs a constant-current control for charging an open/close control terminal of a switching element to be driven which is a voltage-controlled switching element with electric charge for turning on the switching element.
2. Related Art
As a drive circuit for a switching element, there is proposed a circuit that uses a constant-current circuit to charge a gate of an insulated gate bipolar transistor (IGBT) to be driven, as disclosed by, for example, JP-A-2009-011049. The constant-current circuit is configured by using a direct current (DC) voltage source with a large enough terminal voltage as a power supply. This aims at keeping high controllability of constant-current control. The constant-current circuit can increase an upper limit of the gate voltage of the IGBT. Excessive increase in the upper limit may cause decrease in reliability of the gate oxide film of the IGBT. To avoid such a situation, based on the gate voltage of the IGBT, a constant voltage that is lower than the DC voltage source is applied to the gate.
However, when the IGBT to be driven is configured by a series connection of a switching element at a high-voltage side and a switching element at a low-voltage side, if abnormality where feedthrough current flows in the IGBT occurs, a switching to an on-state of the IGBT results in a rapid and excessive increase in current flowing in the IGBT. This leads to increase in the gate voltage of the IGBT, and therefore, prior to a switching to the constant voltage, such excessively large current as to cause decrease in reliability of the IGBT may flow in the IGBT.